1. Field of the Invention
The present invention relates to a solid state image sensing device capable of reducing a power dissipation.
2. Description of the Prior Art
Of solid state image sensing devices, there is known a floating well amplifier (FWA) type solid state image sensing device. In the FWA solid state image sensing device, when holes (positive holes) obtained by photo-electric conversion of a photosensitive element, e.g., photo-diode are accumulated in a P-type well of an N-channel MOS (metal oxide semiconductor) transistor (pixel transistor) and this pixel transistor is turned on, a surface potential of the P-type well is fluctuated by the accumulated holes and a surface channel current is changed. The changed amount of the channel current is read as an output signal of each of the pixels (see FIG. 5).
In this solid state image sensing device, a drain electrode of a load transistor such as an N-channel MOS transistor or the like is connected to the source electrode of the pixel transistor to form a source follower circuit. Thus, when the load transistor is turned on, a channel current is flowed to the P-type well of the pixel transistor in accordance with the source follower circuit.
In the above FWA solid state image sensing device, in order to prevent a blooming from taking place when signal charges accumulated in the P-type well are overflown to adjacent pixels, a gate voltage of the pixel transistor is constantly set to about 5 V and the pixel transistor is set in its on state.
According to the prior art, since the above load transistor is constantly set in its on state, a current is constantly flowed to the source follower circuit formed by the pixel transistor and the load transistor.
Since a current is constantly flowed to the source follower circuit as described above, the conventional FWA solid state image sensing device consumes a large amount of an electric power.